Fig. 4

Photo-thermionic (PTI) effect in vertical graphene-silicon heterojunction. a Low power C.W. photocurrent spectrum of PTI process. Red squares and blue circles represent hybrid and monolithic devices. b Reverse bias voltage-dependent photocurrent characteristics for graphene–Si p-i-n junction under C.W. laser (empty circles) and subpicosecond pulsed laser (solid squares) illumination centered at 1550 nm. Inset: band diagram of PTI on graphene–Si interface and avalanche gain in Si. c Responsivity versus laser pulse power for silicon (red squares) and graphene (blue empty dots) samples with zero bias. The pulsed laser propagating in intrinsic region (PhC waveguide) is centered at 1550 nm. Inset: Photocurrent versus input power for devices with (red squares) and without (empty blue circles) graphene with in-plane excitation under zero bias. The dashed line is a linear relationship to the hybrid device, showing PTI dominant process at 1550 nm. The blue curve is a polynomial fit to the silicon device. d Microwave spectrum of the converted photoelectric signal with a center frequency of 40 GHz. Inset: the zoom-in spectrum of the center peak, with 5 Hz bandwidth and electrical signal-to-noise ratio of 52.9 dB